The present invention relates to an optical scanner for use with laser beam printers and the like.
Imaging lenses for use with optical scanners are required to have two aberrational characteristics. First, they should have a specified negative distortion to achieve scanning at uniform speed; second, they should have a small enough curvature of the field to provide a flat image plane such that the beam spot size is close to a diffraction limited value.
The imaging lens used with conventional optical scanners is composed of one or more elements. If the aberrational characteristics are paramount, an increased number of lens elements are used and a single lens is used if the cost is important. To provide further improved aberrational characteristics, the use of aspheric surfaces on the imaging lens is a recent approach as typically shown in Unexamined Published Japanese Patent Application No. 50908/1992. The imaging lens taught in this reference is characterized in that both the entrance and exit faces have their curvatures in a sub-scanning direction varied continuously over the effective lens area along a main scanning direction. In regard of the entrance face which is symmetric with respect to the optical axis, the curvatures in the main and sub-scanning directions are dependent on each other.
To achieve high-speed scanning, Unexamined Published Japanese Patent Application No. 100742/1976 proposed that a semiconductor laser array having a plurality of independently modulative light-emitting portions be used as a light source so that more than one scanning line could be produced simultaneously by one scanning cycle.
However, an optical scanner using such a semiconductor laser array had the problem that the beams applied formed unevenly spaced scanning lines.
With a view to solving this problem, Unexamined Published Japanese Patent Application No. 158251/1979 proposed that the plurality of light-emitting portions in the semiconductor laser array be arranged at an angle with the main scanning direction such that the variation in the spacing between scanning lines could be reduced.
Another approach was proposed by Unexamined Published Japanese Patent Application No. 161566/1981, which ensured a constant spacing between scanning lines by locating a cylindrical lens away from the surface to be scanned by a distance substantially equal to the focal length of the lens.
The spacing between scanning lines can also be made constant by providing an ultrasonic optical deflector between a light source and a rotating polygonal mirror as taught in Unexamined Published Japanese Patent Application No. 21031/1985; this approach is based on the correction of the curvature of scanning lines by adjusting the Bragg diffraction angle.
According to yet another proposal made by Unexamined Published Japanese Patent Application No. 54211/1990, the lateral magnification of optics ranging from the light-emitting portions to the surface to be scanned is adjusted to be no more than 2 in absolute value such that the spacing between scanning lines that are produced simultaneously is sufficiently reduced to minimize the variation in that spacing.
These proposals, however, have had their own problems. Speaking of the imaging leans taught in Unexamined Published Japanese Patent Application No. 50908/1992, the optical magnification in the sub-scanning direction varies within the effective scanning region, so the diameter in the sub-scanning direction of a beam spot that is formed on the surface to be scanned becomes uneven within the effective scanning region. If an optical scanner incorporating such an imaging lens is used with a laser beam printer, the print density obtained is so uneven that one cannot produce satisfactory print quality.
Referring to the optical scanner that employs a semiconductor laser array as a light source to achieve simultaneous production of more than one scanning line by a single scanning cycle, the approach taught in Unexamined Published Japanese Patent Application No. 158251/1979 has the disadvantage that the variations in the spacing between scanning lines cannot be completely corrected. In addition, if the number of beams is increased, there occurs a corresponding increase in the amount of variation in the spacing between scanning lines. Therefore, the number of beams cannot be increased to such a value that the advantage of performing high-speed scanning with a plurality of beams is exhibited to the fullest extent.
The approach proposed in Unexamined Published Japanese Patent Application No. 161566/1981 requires not only a plurality of semi-conductor lasers but also as many mirrors and cylindrical lenses as the beams to be produced. This adds greatly to the cost of the overall system and increases the complexity and size of the optics, thereby making the system far from being practically feasible.
The ultrasonic optical deflector taught in Unexamined Published Japanese Patent Application No. 21031/1985 is not only expensive, complicated and bulky but also requires a complicated drive circuit.
In the proposal made by Unexamined Published Japanese Patent Application No. 54211/1990, the lateral magnification is as low as 2 or less, so most of the optical output of the semiconductor laser is lost by "vignetting" and only a small part of the output reaches the surface to be scanned, with the result that the optics can achieve a very low efficiency in optical output. Hence, the optical output of the semi-conductor laser is insufficient for practical purposes. What is more, the variation in the spacing between scanning lines is not completely corrected but merely reduced in proportion to the decrease in the lateral magnification and the problem at issue cannot be completely solved.